1. Field of the Invention
The present invention relates to a method for manufacturing a glass. More particularly, the present invention is concerned with a method for manufacturing, through a porous body, a multi-component glass, especially a glass for use as a gradient index optical element, such as optical lenses for cameras, microscopes and the like.
2. Discussion of Related Art
Various methods have been proposed for manufacturing a multi-component glass from a porous body. For example, the multi-component glass may be manufactured according to the sol-gel method in which use is made of a porous gel produced by subjecting a metal alkoxide to hydrolysis and a polycondensation reaction or a porous gel is produced by dispersing water glass or a particulate of an oxide, such as SiO.sub.2, in a solvent to obtain a sol and regulating the pH value of the sol (see Japanese Patent Application Laid-Open Specification No. 277525/1988). Alternatively, the multi-component glass is manufactured by a method in which use is made of a porous glass produced by melting an alkali borosilicate glass, and conducting thermal treatment for phase separation and acid treatment to leach out a soluble phase (see Japanese Patent Application Publication No. 21173/1986). Still alternatively, the multi-component glass is manufactured by a method in which use is made of soot produced by depositing metal oxide particles according to, for example, the CVD technique [see Hikaligijutsu Contact (optical technology contact), 28 (1990), 228-232].
In particular, a brief description will be made with respect to the steps for manufacturing, through a porous body, a homogeneous multi-component glass or a glass having a distribution of refractive index, etc. First, a porous body containing a metal component is produced, and the metal component is fixed while it has no concentration distribution. Drying and sintering converts the porous body to a homogeneous glass. If the metal component (ion) is fixed after providing the same with a concentration distribution and then the porous body is dried and sintered, a glass having a distribution of refractive index, etc. results.
The term "fixing" used herein refers to a step of precipitating, as microcrystals, a metal salt present in dissolved form in the pores of a porous body to thereby prevent the moving of the metal salt in the pores. A method utilizing a solubility difference has been employed for the fixing of a metal component in a porous body. In particular, the following procedure has been proposed, depending on the strength and properties of the porous body.
When a phase separation glass is used, it is immersed in an aqueous solution of a metal nitrate, as a dopant, heated at about 100.degree. C. to thereby stuff the pores thereof with the metal nitrate (stuffing step). Subsequently, the glass is immersed in a solution capable of diffusing the metal nitrate (e.g., ethanol/water solution heated at about 70.degree. C.) to gradually leach out the solution containing the dopant metal nitrate outside the phase separation glass (unstuffing step) so that a concentration distribution is provided. The resultant porous body with the concentration distribution is immersed in ethanol kept at about 0.degree. C. in which the solubility of the nitrate is low to precipitate microcrystals in the pores of the porous body so that the concentration distribution is fixed. Thereafter, the porous body is dried and subjected to thermal treatment to obtain a dense glass. With respect to the method for manufacturing a glass with a distribution of refractive index according to the molecular stuffing method as set forth above, reference is made to Japanese Patent Application Publication No. 21173/1986.
Japanese Patent Application Laid-Open Specification No. 277525/1988 proposes a method for manufacturing a glass, comprising preparing a porous gel from a raw material of a solution containing an alkoxide of Si as a major component and a water soluble salt of a specific metal ion and subjecting the porous gel to drying and sintering according to the sol-gel method, wherein the porous gel is immersed in an organic solvent miscible with water, in which the solubility of the metal salt is low, to introduce the metal, thereby obtaining a multi-component glass.
Further, Japanese Patent Application Laid-Open Specification No. 295818/1991 discloses a method for manufacturing a glass body provided with a distribution of refractive index using a metal salt according to the sol-gel method, in which a fixing operation is applied to a porous body.
This method will briefly be described below.
An aqueous solution of lead acetate as a metal component for providing a refractive index distribution (first metal component) is added to a sol prepared using a silicon alkoxide as a raw material for skeleton-forming oxide, thereby producing a cylindrical gel. This gel is successively immersed, for the utilization of a difference in solubility of lead acetate, in solvents each comprising acetone and isopropanol (often referred to as "IPA") in a mixing ratio having staged variation from 0 to 1 to cause staged decrease in the solubility, so that microcrystals of lead acetate are precipitated on the wall of the pores of the gel. Thus, lead acetate is fixed. If the gel is immersed in acetone in which the solubility of lead acetate is low in place of the solvents having a mixing ratio with staged variation, the precipitation of lead acetate is so rapid that crystals grow in the gel to destroy the skeleton of the gel.
Subsequently, the gel is immersed in a distribution-providing solution comprising ethanol and, dissolved therein, potassium acetate as a second metal component for replacing the lead acetate in the gel. In this immersion step, counter diffusion of lead and potassium ions occurs to thereby form a convex profile in the lead acetate concentration in the radial direction and a concave profile in the potassium acetate concentration in the radial direction. The potassium of the second metal component is introduced in a distribution profile reverse to that of lead so that a distributional change of thermal expansion coefficient (often referred to as ".alpha.") caused by the concentration distribution of lead is compensated for. The formed distribution profile can be controlled by the period of immersion in the distribution-providing solution, etc. The resultant gel is immersed in solvents each comprising acetone and IPA in a mixing ratio having staged variation to thereby precipitate microcrystals of lead and potassium acetates on the wall of the pores of the gel. Thus, the concentration distributions of the metals are fixed in the skeleton of the gel.
The resultant wet gel is dried and sintered, thereby obtaining an optical element having a refractive index distribution in the radial direction.
With respect to an optical element having a refractive index distribution, i.e., gradient index optical element, it is known that a plurality of metal components must be provided with concentration distributions with combinational concave and convex profiles in order to ensure highly effective optical design (see U.S. Pat. No. 5,166,827).
However, the glasses manufactured by the above-mentioned conventional processes have two major drawbacks.
The first drawback resides in that the fixing of the metal component incorporated in a porous body is not certain, so that a multi-component glass with a desired composition cannot be obtained. This is also true with respect to a glass having a metal component provided with a distribution. That is, the fixing of the provided distribution profile of the metal component is not certain, so that a glass with a desired distribution profile cannot easily be obtained.
The second drawback resides in that, in providing a plurality of metal components with distributions, it is not feasible to individually control the distribution profiles of metal components, so that various characteristics attributed to the distribution of the composition of the glass (e.g., distributions of diffusion and thermal expansion coefficient) cannot be controlled.
These two drawbacks will further be described in greater detail.
Now, the first drawback relating to the fixing of metal components will be described.
In the method using a water soluble metal salt as disclosed in Japanese Patent Application Laid-Open Specification No. 277525/1988, use is made of a salt, such as an acetate, a nitrate and a chloride. When a porous gel is immersed in an organic solvent miscible with water, in which the solubility of the salt is low, in order to fix such a metal component in the porous gel, however, a portion of the metal salt in the porous gel is precipitated as microcrystals and fixed in the pores of the porous gel, as mentioned above, while the rest of the metal salt is leached out from the porous gel, in a form dissolved in the solvent in the pores of the porous gel. The solubility of the metal salt is lower in the organic solvent than in the solvent of the solution in the porous gel, so that the metal salt leached out is precipitated in the organic solvent. In the case where the organic solvent is replaced by a solvent of a type, and with a mixing ratio selected to increase the solubility of the metal salt for the purpose of inhibiting crystal precipitation in the organic solvent, the metal salt is leached out from the porous gel, in a form dissolved in the replacing solvent. Thus, the absolute amount of the metal salt fixed and remaining in the porous gel is decreased.
Introduced metal elements include those, such as rare earth elements, which give an acetate salt having low solubility in water or an alcohol. When an acetate of a rare earth element is employed, the solubility thereof cannot be sufficient so that the acetate cannot be incorporated in the porous gel in a desirably large amount, or that, in previously doping a sol with a metal component according to the sol-gel process, a large amount of solvent is needed so that the sol is excessively diluted. Thus, cracks due to lowered gel strength frequently occur, and hence, it has been difficult to manufacture a glass with a desired morphology.
The acetate may be replaced by a nitrate or a chloride. In the method as disclosed in Japanese Patent Application Laid-Open Specification No. 277525/1988, the porous body is immersed in an organic solvent miscible with water to precipitate and fix microcrystals. In the immersion of a nitrate or a chloride in an organic solvent in which the solubility of the salt is appropriate, a portion of the salt is fixed, while the rest is leached into the immersion solvent. Thus, it is difficult to completely fix the metal salt in the porous body, so that the manufacturing of a glass with a desired composition has been difficult.
That is, with respect to rare earth elements and other metal species for which the use of a nitrate, a chloride or the like is inevitable, it has been unfeasible to effect fixing while a desirably large amount of metal remains in the gel irrespective of the use of the type of employed solvent (although the degree of crystal precipitation depends on the type of immersion solvent), as long as the conventional technology is employed.
Further, the step which is inevitable in the manufacturing of a gradient index optical element according to the method proposed in Japanese Patent Application Laid-Open Specification No. 295818/1991, etc., is one for providing the lead acetate in a porous gel with a concentration distribution. However, as mentioned above, a portion of the lead acetate is present as microcrystals in the pores of the porous gel, while the rest is present in the pores in a form dissolved in the distribution-providing solution, in the distribution-providing step. The provision of the distribution is governed by the permeation of the distribution-providing solution into the porous gel and by the mutual dissolution and counter diffusion of lead and potassium acetates. At this stage, it is believed that the distribution profile of lead acetate is as shown in FIG. 1, has neither extremum nor inflection point in the radial direction, and exhibits a monotonous decline from the center toward the periphery. When such a wet gel is immersed in a solution in which the solubility of lead acetate is low, the lead acetate in the pores is gradually precipitated as microcrystals in accordance with the permeation of the solution. At this stage, the pore size distribution of the wet gel ranges from about ten to several hundreds of angstroms, in which the moving rate of the solution is small in the pores of the gel. Hence, the exchange between the solution in the pores and the solution outside the gel slowly proceeds. During that period, the lead acetate dissolved in the solution in the pores of the gel is conditioned to be capable of easily migrating toward the periphery of the gel.
Moreover, when the fixing operation of the lead acetate provided with a concentration distribution is poor, it further migrates toward the periphery, due to a slight dissolution in the solution for fixing present in the pores of the wet porous gel, in accordance with the evaporation of a solvent during the step of drying the gel, thereby being deposited at the periphery.
On the other hand, since this fixing operation causes microcrystals to gradually precipitate in the porous gel, the solvent exchange step must be performed in multiple stages, so that a large volume of solvent is required as an immersion liquid, and that a prolonged period of time is consumed to run through the whole step. The prolonged immersion of the porous gel disadvantageously causes the concentration distribution provided for the metal salt to readily break in the fixing step after distribution provision. In order to avoid this disadvantage, a measure, such as changing mixing proportions of solvents to bring about a lowered solubility, has been tried. However, the leaching of the metal salt outside the porous gel cannot be avoided, and crystal precipitation is observed in the fixing liquid due to the lowered solubility. Thus, no improvement is attained with respect to the break of the concentration distribution.
The above causes the concentration distribution of lead acetate in the gel produced by the method of Japanese Patent Application Laid-Open Specification No. 295818/1991 to have an inflection point and an extremum in the periphery and a low concentration in the center, so that the concentration difference is small between the periphery and the center.
The resultant xerogel is sintered to obtain a glass. The refractive index distribution of the thus obtained glass accords with the concentration distribution profile of lead, which exhibits a refractive index lower than a desired value in the center to thereby cause the refractive index difference (often referred to as ".DELTA.n") to be small between the center and the periphery and also exhibits an extremum and an inflection point in the radial direction (see FIG. 2). From the viewpoint of optical design, the glass only finds very limited use and application.
When a gradient index optical element is used in a lens system for a camera, etc., the distribution profile of refractive index thereof is important.
With respect to the gradient index optical element, the following relationship is observed between the refractive index and the distance from the center. EQU N(r)=N.sub.0 +N.sub.1 r.sup.2 +N.sub.2 r.sup.4 +N.sub.3 r.sup.6 +(1)
N(r): refractive index at a radius r from the center of the element PA1 N.sub.0 : refractive index in the center PA1 N.sub.1, N.sub.2, N.sub.3 . . . : distribution coefficient.
What is important with respect to the distribution profile is .DELTA.n and the distribution coefficient in the lens periphery, which concerns correction of chromatic aberration. (The power of the gradient index optical element depends on the value of N.sub.1 when the influences of the coefficients N.sub.2 and N.sub.3 are small. The greater the value of N.sub.1, i.e., the greater the value of .DELTA.n, the greater the effect of the gradient index optical element. For the chromatic aberration correction in the lens periphery, the distribution coefficients N.sub.3 et seq. are important, which have marked influence on the distribution profile in the periphery.)
Fitting into formula (1) of the refractive index values at various points in the radial direction of the glass manufactured by the method disclosed in Japanese Patent Application Laid-Open Specification No. 295818/1991 reveals that the value of N.sub.3 is large, with a distribution profile having a reversion of convex and concave refractive index profiles in the periphery, and that the refractive index in the center is low so that .DELTA.n is as small as 0.070 (refer to FIG. 2).
When a lens system is prepared using the element with the above distribution profile, not only the value of .DELTA.n is so small that the power of the medium per se is weak, but also an inflection point exists in the periphery of the lens, so that light cannot be satisfactorily condensed to thereby cause the effect of chromatic aberration correction characteristic of the gradient index optical element to be unattainable. The trial to obviate the inflection point in the periphery disadvantageously causes the lens to have an only limited effective diameter.
The proportion of the amount of the metal salt fixed in the pores of the porous gel to that of the metal salt leached out from the gel depends on the type and quantity of the metal salt, and so cannot be equally stated. However, at any rate, there is a limit in the method of Japanese Patent Application Laid-Open Specification No. 295818/1991 comprising changing the mixing ratio of solvents to precipitate microcrystals of a metal salt with the use of solubility difference only. Experimental study has revealed that desired effect cannot be attained, depending on the type of introduced metal and the required fixing precision (e.g., secure retention of a distribution profile in the manufacturing of a gradient index optical element).
Similar problems are encountered by the manufacturing of a glass provided with a refractive index distribution according to the generally known molecular stuffing method. In this method, for incorporating a large amount of metal component, a nitrate having a high solubility is used, and a fixing is performed with a solvent in which the solubility of the metal component is low, at a temperature difference of about 100.degree. C. However, the nitrate generally has high solubility in a polar solvent, such as water and an alcohol. For example, the nitrate has a solubility of about 1 g/100 ml even in ethanol cooled to about 0.degree. C. Thus, it is difficult to completely fix the metal in the porous body, and the metal salt dissolved in the fixing solution migrates during the drying step, so that a concentration distribution cannot be provided with desired precision. Moreover, it is difficult to cause the .DELTA.n to have a large value.
The second major drawback relating to individual control of a plurality of metal components will now be described in detail.
The glass manufactured by the method of Japanese Patent Application Laid-Open Specification No. 295818/1991, besides the above-mentioned problem of the break of the metal distribution profile, suffers from cracks on the surface thereof, which render the glass unsatisfactory as an optical element.
The cracks are attributed to a distribution of .alpha. caused by a composition distribution along the radial direction. It is believed that the trial to compensate for the convex profile distribution of .alpha. due to the distribution of lead by the concave profile distribution of potassium is unsuccessful.
In this respect, the composition distribution of the prepared glass has been analyzed to obtain FIG. 3. The .alpha. distribution along the radial direction has roughly been estimated on the basis of the results as shown in FIG. 3 to obtain FIG. 4. A concave distribution profile is shown therein, which exhibits a small value of .alpha. in the center with a large value in the periphery because of overcorrection of .alpha. by potassium. It is believed that the concave profile distribution of .alpha. causes the radial shrinkage of the gel during the sintering step to suffer from a strain, which causes the glass to suffer from cracks.
Noting potassium concentrations as shown in FIG. 3, rendering the concentration difference between the center and the periphery insignificant would allow the .alpha. values as shown in FIG. 4 to be constant along the radial direction. For rendering the .alpha. values constant along the radial direction without changing the distribution profile of refractive index, it is believed that the concentration of a potassium salt in a distribution-providing liquid should be decreased while the time for distribution provision should remain unchanged. However, this is not effective for avoiding cracks. The composition distribution profile of the resultant glass has been studied to obtain FIG. 5. As indicated by solid lines therein, although the desired distribution profile has been realized with respect to potassium, the phenomenon has been found that it is accompanied by a change in the distribution profile of lead.
Shortening the distribution-providing time would be contemplated to avoid cracks. Actually, however, simply changing the time is not effective for avoiding cracks, and the disadvantage arises that the concave and convex metal composition distribution profiles are simultaneously changed to cause the desired refractive index distribution to be unattainable. In any way, it has been unfeasible to obtain an optical element with the desired distribution profile of refractive index.
Moreover, in the above conventional methods, it is unfeasible to individually control the concave and convex distribution profiles of metal components, as mentioned above. Hence, despite the possibility of the manufacturing of an optical element highly effective from the viewpoint of optical design if the composition distributions are provided with desirable concave and convex profiles, actually, it has been unfeasible to manufacture such an optical element.